Is it legal to cross an internal Schengen border without passport for a day visit

Schengen rules kind of dance around this issue. The Schengen acquis does not create or remove any specific requirement to hold documents to cross internal borders. It also explicitly outlaws border control (with a few nuances and caveats) so if holding a specific document was required, that requirement could not legally be enforced, at least not systematically.

In fact, I suspect that historically in many places, what’s illegal is crossing outside of an official border check point. And if you cannot identify yourself at an official border checkpoint, you would typically be turned away, making the question moot. In these circumstances, even where there is an administrative requirement to hold some specific document, it’s not necessarily a separate criminal offence to cross without it but rather something that was meant to be checked by border guards.

At the same time, many Schengen countries (including, it seems, Austria) have strict ID and registration requirements even while residing in the country. The Schengen Borders Code explicitly allows that in article 23(c) and 23(d). This makes the question rather irrelevant in another way: If you come to the attention of the police and they really want to create trouble for you, you are just as likely to be held, fined, or prosecuted under these laws than punished specifically for having crossed the border.

By contrast, in a country where there is no such requirement (e.g. France), you would not be committing a specific offence by crossing the border without your passport and I don’t see anything illegal about it. Finally, note that proximity to a border can be legitimate grounds for checks that would otherwise be technically illegal (it’s explicitly the case in France and I think in Germany too). So actually crossing the border might not even be required to give the police cover to ask you to identify yourself and ultimately book you for not having your passport on your person (where required) or even a completely unrelated offence stemming from that check.

This caused a bit of tension in the group as there was disagreement on whether this is in fact unlawful or not.

You were right, they were wrong.

Both Germans and non-Germans are required to carry a valid travel document with them when crossing the german border.

Since 1957, many national ID cards are considered equal to passports as far as the conditions of §13 AufenthG and §1 PaßG are concerned.

Did I spoil the idea for no good reason?

People have short memories and they tend to forget that even in the early 1970’s one could spend hours at both the Austrian and Italian borders when crossing by car (the same for Belgium).

Just because there is no mandatory checks at the border doesn’t mean that the requirement no longer exists to carry a valid travel document with you.

germany

§13 – Border crossing – (AufenthG)
(1) Entry into and exit from the federal territory is permitted only at the approved border crossing points and within the stipulated traffic hours, unless exceptions are permitted on the basis of other statutory provisions or intergovernmental agreements. When entering or leaving the federal territory, foreigners are required to carry a recognised and valid passport or passport substitute as referred to in section 3 (1) and to submit to the police control of cross-border traffic.

§1 – Passport requirement – (PaßG)
(1) Germans within the meaning of Article 116 (1) of the Basic Law of the Federal Republic of Germany leaving or entering the geographical area in which this law applies are required to carry a valid passport to identify themselves. Presenting a passport of the Federal Republic of Germany within the meaning of (2) shall fulfil this requirement.

austria

§15. Voraussetzung für die rechtmäßige Einreise in das Bundesgebiet – Fremdenpolizeigesetz 2005
(1) Fremde brauchen, soweit durch Bundesgesetz oder durch zwischenstaatliche Vereinbarung nicht anderes bestimmt ist oder nicht anderes internationalen Gepflogenheiten entspricht, zur rechtmäßigen Einreise in das Bundesgebiet ein gültiges Reisedokument (Passpflicht).

Requirement for lawful entry into the federal territory
(1) Foreigners need a valid travel document (passport obligation) to lawfully enter the federal territory, unless otherwise stipulated by federal law or an international agreement or in accordance with international practice.

§2. Ausreise und Einreise – Passgesetz 1992
(1) Österreichische Staatsbürger (Staatsbürger) bedürfen zur Ausreise aus dem Bundesgebiet und zur Einreise in dieses eines gültigen Reisedokumentes (Reisepaß oder Paßersatz), soweit nicht etwas anderes durch zwischenstaatliche Vereinbarungen bestimmt wird oder internationalen Gepflogenheiten entspricht. …

(1) Austrian citizens (citizens) require a valid travel document (passport or passport substitute) to leave and enter the federal territory, unless otherwise determined by intergovernmental agreements or international customs. …

• European Agreement on Regulations governing the Movement of Persons between Member States of the Council of Europe (ETS No. 025)
  • ETS 25 – Movement of Persons (Annex), 13.XII.1957 (PDF)
    • List of documents, per country

In theory, you did the right thing as you’re theoretically meant to hold a travel document to cross the internal border.

In reality, you were WAY overthinking it. Anywhere you can cycle over the border, enforcement will be all but non-existent. While Austria theoretically requires foreign nationals to be able to identify themselves to the authorities, again, back in reality this would only become relevant if, for instance, checking in at a hotel or involved in an accident/crime